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Correct size sched 40

I have installed a couple small sprinkler systems with no resulting problems. Now a neighbor has asked me to help him with a good sized job at his house. He has 100 psi on city water (haven't measured flow yet), with a 1" T off the line from the meter. Unfortunately, he had workers building a retaining wall, and paving the driveway run a 3/4" PVC line under them (would have been nice to have 1"). Since we are going to lay about 120' of line on the other side of the driveway and retaining wall to a couple valves, then use rotors on these valves, my question is: is it better to use 1" pvc after we've used 3/4 under the driveway? When the 3/4 has squeezed the flow, is there any use to go back up to 1"

It's all just numbers. Check the pressure loss charts for the flow rates you will have. If there was any point in having 1" pipe in the first place, then you will want to have it on both sides of the driveway.

You have to consider the run of 3/4 as just a speed bump on the one inch highway. It does not force you into anything. I have fed entire sprinkler systems through a 1/2 inch pipe nipple, when it was the only logical way to proceed.

(Note: The edits have modified some of my numbers, I was reading from the wrong pressure loss chart).

Yes you can re-upsize to 1" after being "squeezed" through the 3/4". You're just going to be losing about 3-16 psi (depending upon your flow rate) through the 3/4". By increasing back up to 1", you'll reduce the amount of additional friction losses. Given that you're starting with 100psi, the pressure loss through the 3/4" alone donesn't seem to be a big deal.

Where you'll have more to worry about is in water velocity. The general rule is that you don't want the water flow to exceed 5 feet/sec. For a 3/4" Sch40 pipe, that would limit you to a flow of 8gpm (and about a 7psi pressure loss in 15' of 3/4" Sch40). At 12gpm (max flow recommended for 1" Sch40), the water velocity through the 3/4" pipe would be 7.2 feet/sec (44% above recomened max, and about 16psi pressure loss in 15' of 3/4" Sch40). That could lead to water hammer and increased stress on the system (especially since you're starting with 100psi).

If you want a flow rate at or below 8gpm, you need to only take the 7psi(+/-) pressure lost into account (doesn't sound like a big deal since you're starting with 100psi). But if you want higher flow rates, then you're in sort of a catch-22. One of the general rules of thumb I've heard regarding sprinkler systems is that if your starting pressure is greater than 85psi, you should consider installing a pressure regulator to get it below 85psi. But if you do that and design for a 12gpm flow, then your working pressure is already down to 69psi (85psi - 16psi for the 3/4" pipe) and we have not accounted for pressure losses through valves, backflow preventer, and other friction losses in the rest of the pipe). But if you don't include a pressure regulator, then add to the existing issue of water hammer the fact that it can be occuring at high pressures. And water hammer effects the entire system, not just the 15' of 3/4" pipe.

Now I'll admit that I have NO practical experience with this (just passing on the "book learning" I've obtained researching this subject).

Here's an Idea: 3/4" Sch40 PVC has about the same OD as 1" copper. Would it be possible to pull out the PVC and push in a replacement copper?